The identification of ore/waste boundaries is a common, and, usually necessary, part of recovering valuable minerals as part of the mining process. It serves two primary purposes: firstly, it ensures that ore loss is minimised at the excavation stage; secondly, it ensures that the treatment of waste is minimised in the post-mining recovery stage. Of course, the initial stage of blasting is designed to minimise mixing between the two components (ore and waste) and reduce ore body sterilisation.
The issue is tackled on a daily basis at all mine operations globally. Simple calculations indicate significant impact on mine profitability but the actual tracking of these ore/waste boundaries is difficult and time-consuming. Mines often accept a level of ore loss and factor this into their financial analyses and predictions.
Current methods for tracking these boundaries usually involve a grid of assay data, often obtained from each blast hole, although the scale of the boundaries and the ore-body geology influence the nature of the assaying demands. Physical targets have been used to track the boundaries after blasting. These targets include visual markers such as PVC pipes installed in extra boreholes within and along the boundaries, or coloured sandbags; magnetic metal targets such as metal balls, chains and the like that are picked up using simple metal detectors. Nuclear markers have also been proposed.
The most attractive techniques are those that enable the excavator operator to make decisions at the time of digging based on whether the current dipper load is ore and is meant for the mill or whether it is waste and is meant for the waste dump. None of the approaches described above have this benefit. In some mines a spotter is required to assist the operator to make that decision—a further, albeit small, cost impost on the operation.
A recent technique is the use of self-righting radio transmitters placed within witness boreholes along the ore-waste boundary, discussed in Australian patent application 2004202247 and in a related paper by Thornton et al “Measuring Blast Movement to Reduce Ore Loss and Dilution”, International Society of Explosives Engineers, 2005G, Vol. 2, 2005. An antenna is walked across the post-blast muckpile and the radio transmitters are detected by their signal strength. The method works well but is not well integrated into the normal mine activities.
A somewhat similar technique, described in Firth, I R et al (2002), ‘Blast movement measurement for grade control’, Proc. 28th ISEE Annual Blasting Conference, Las Vegas, February 10-13, utilises square section magnetic targets attached at the end of a steel section of 300 mm length. A magnetometer is walked across the post-blast rock and peaks in the signal are detected. The targets give an accuracy of about 0.6 m in the horizontal plane. Reference is also made to a paper by Taylor et al “Utilisation of blast movement measurements in grade control”, Application of Computers and Operations Research in the Minerals Industries, South African Institute of Mining & Metallergy, 2003, 243-247. This paper outlines a method for delivering data post-blast from an array of magnetic targets.
It is to be understood that any reference herein to prior utilised or disclosed techniques is not to be taken as an admission that those techniques constitute part of the common general knowledge, whether in Australia or elsewhere.
It is an object of the invention to provide one or more methods of mining mineral deposits that include aspects adaptable to facilitate post-blast boundary location or other characterisation of a deposit.